The evolution of sedimentary environments of Dongyuemiao Member and key factors for enrichment of shale oil and gas, northeastern Sichuan Basin

The organic-rich mud shale develops in Dongyuemiao Member of the Lower Jurassic Ziliujing Formation in eastern Sichuan Basin. Although a few of industrial shale oil and gas flow has been obtained, the large-scale production of shale oil and gas hasn't started yet, and the understanding of the enrichment mechanism of shale oil and gas in the region is not enough. Based on the field investigation and test analysis, this paper analyzes the evolution of sedimentary environments of the Dongyuemiao Member and discusses the main control factors for enrichment of shale oil and gas, in order to provide scientific suggestions for the breakthrough of shale oil and gas in the region. The results show that:(1) The Dongyuemiao Member had experienced a whole retrogradation-progradation cycle, and the organic-rich shale, which is in the semi-deep lake facies, is located at the middle of the Dongyuemiao Member; (2) The thickness of organic-rich shale in the Dongyuemiao Member is from 15 m to 30 m and its average TOC is 1.79%, and the kerogen is mainly in II type; (3) The thermal evolution degree of the Dongyuemiao Member is high, reaching condensate-moisture stage; (4) The pores are mainly composed by the soil mineral intergranular pores and organic pores, and the content of brittle minerals is moderate and the average brittleness index is 62.1% and the average porosity is 8.85%; (5)The permeability is extremely low, with an average of 0.239 mD. It is concluded that the Dongyuemiao Member belongs to the "high porosity and low permeability". The hydrocarbon in the Dongyuemiao Member is mainly migrated along the bed plane in lateral directions. The enrichment of the shale gas and oil in Dongyuemiao Member is closely related to the distribution of organic-rich shale, the thermal evolution of the shale, and the distribution of fractures. The prolific zones for shale gas and oil in Dongyuemiao Member are characterized with thick organic-rich shale and low hydrocarbon accumulation potential.